You should be able to use MAPI Extended Properties:
Appointment state is
Named Prop Name: id: 0x8217=33303 = PidLidAppointmentStateFlags, dispidApptStateFlags
Named Prop Guid: {00062002-0000-0000-C000-000000000046} = PSETID_Appointment
So SOAP that should be something like
<t:ExtendedProperty>
<t:ExtendedFieldURI DistinguishedPropertySetId="Appointment" PropertyId="33303" PropertyType="Integer" />
<t:Value>1</t:Value>
</t:ExtendedProperty>
(I use the managed API for that, and got that XML from the trace-log, hope that is something you can use)
[Edited by the OP] This is the complete call that does the job:
<soapenv:Envelope
xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"
xmlns:typ="http://schemas.microsoft.com/exchange/services/2006/types"
xmlns:mes="http://schemas.microsoft.com/exchange/services/2006/messages">
<soapenv:Header>
<typ:RequestServerVersion Version="Exchange2007_SP1"/>
<typ:MailboxCulture>en-US</typ:MailboxCulture>
<typ:TimeZoneContext>
<typ:TimeZoneDefinition Id="W. Europe Standard Time"/>
</typ:TimeZoneContext>
</soapenv:Header>
<soapenv:Body>
<mes:UpdateItem ConflictResolution="AutoResolve" SendMeetingInvitationsOrCancellations="SendOnlyToChanged">
<mes:ItemChanges>
<typ:ItemChange>
<typ:ItemId <t:ItemId Id="AAMkA[snip]xAAA=" ChangeKey="Dw[snip]Mar"/>
<typ:Updates>
<typ:SetItemField>
<typ:ExtendedFieldURI DistinguishedPropertySetId="Appointment" PropertyId="33303" PropertyType="Integer"/>
<typ:CalendarItem>
<typ:ExtendedProperty>
<typ:ExtendedFieldURI DistinguishedPropertySetId="Appointment" PropertyId="33303" PropertyType="Integer"/>
<typ:Value>5</typ:Value>
</typ:ExtendedProperty>
</typ:CalendarItem>
</typ:SetItemField>
</typ:Updates>
</typ:ItemChange>
</mes:ItemChanges>
</mes:UpdateItem>
</soapenv:Body>
</soapenv:Envelope>
Strangely, the result says "Success", but reports a (one) conflict. I have no idea which one:
<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
<s:Header>
<h:ServerVersionInfo MajorVersion="15" MinorVersion="1" MajorBuildNumber="225" MinorBuildNumber="19" Version="V2_48" xmlns:h="http://schemas.microsoft.com/exchange/services/2006/types" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/>
</s:Header>
<s:Body>
<m:UpdateItemResponse xmlns:m="http://schemas.microsoft.com/exchange/services/2006/messages" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types">
<m:ResponseMessages>
<m:UpdateItemResponseMessage ResponseClass="Success">
<m:ResponseCode>NoError</m:ResponseCode>
<m:Items>
<t:CalendarItem>
<t:ItemId Id="AAMk[snip]xAAA=" ChangeKey="DwA[snip]aMat"/>
</t:CalendarItem>
</m:Items>
<m:ConflictResults>
<t:Count>1</t:Count>
</m:ConflictResults>
</m:UpdateItemResponseMessage>
</m:ResponseMessages>
</m:UpdateItemResponse>
</s:Body>
</s:Envelope>
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